Depth‐Dependent Post‐Treatment for Reducing Voltage Loss in Printable Mesoscopic Perovskite Solar Cells

Abstract The printable mesoscopic perovskite solar cells consisting of a double layer of metal oxides covered by a porous carbon film have attracted attention due to their industrialization advantages. However, the tens‐of‐micrometer thickness of the triple scaffold leads to a challenge for perovskite to crystallize and for the charge carriers to separate and travel to the electrode, which limits the open circuit voltage (V OC) of such devices. In this work, a depth‐dependent post‐treatment strategy is demonstrated to synergistically passivate defects and tune interfacial energy band alignment. Two thiophene derivatives, namely 3‐chlorothiophene (3‐CT) and 3‐thiophene ethylenediamine (3‐TEA), are selected for the post‐treatment. Energy‐dispersive X‐ray spectroscopy proves that 3‐CT is uniformly distributed throughout the triple scaffold and effectively passivates the defects of the bulky perovskite, while 3‐TEA reacts rapidly with the loose perovskite in the carbon layer to form 2D perovskite, forming a type II energy band alignment at the perovskite/carbon interface. As a result, the defect‐assisted recombination is suppressed and the interfacial energy band is regulated, increasing the V OC to 1012 mV. The PCE of the devices is enhanced from 16.26% to 18.49%. This depth‐dependent post‐treatment strategy takes advantage of the unique structure and provides a new insight for reducing the voltage loss.

Growth of (3-TEA) 2 PbI 4 single crystal: 0.4464g PbO was added to a mixed solution of 6mL HI and 0.5mL H 3 PO 2 , heated and stirred at 60 °C until complete dissolution, and then 0.32734g 3-TEACl was added the clarified solution, heated and stirred continuously at 100 °C for 6h and then cooled naturally to room temperature, and the crystallized orange crystals are filtered and dried to obtain (3-TEA) 2 PbI 4 single crystals.
Device fabrication: The FTO glass was first laser etched to get the electrode pattern, then cleaned with detergent, deionized water and ethanol ultrasonically in turn.
The dense layer was deposited on the FTO by spray pyrolysis of titanium diisopropoxide bis(acetylacetonate) at 450 ℃. The m-TiO 2 was fabricated by screen-printing using diluted titanium dioxide paste (1:5 in terpineol) onto the dense layer and sintered at 500℃ in air for 40 min. The m-ZrO 2 and porous carbon electrode were sequentially fabricated by screen-printing onto m-TiO 2 and sintered at 400 °C for 40 min. After the triple-layer scaffold cooling to room temperature, the precursor solution was drop-cast and penetrated in the scaffold, and then annealed at 100 ℃ for 30-60 min. The molar concentration of PbI 2 in the precursor solution was defined as 1. where is Boltzmann' constant, is the temperature of device, is elementary charge, is light intensity, A is a constant, is the diode ideal factor. It is generally considered that =1 represents a second-order (bimolecular) radiative recombination process and =2 represents a first-order (unimolecular) nonradiative recombination process, such as defect-assisted non-radiative recombination [1] .
The incident photo-to-current conversion efficiency was measured by a 150 W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 74004) as a monochromatic light source. Capacitance-voltage (C-V) was performed with ZAHNER Zennium Electrochemical Workstation in dark with a voltage range from 0.1 to -1.2 V at a reserve scan direction with the AC perturbation of 10 mV and frequency of 20 kHz.
The C-V data were analyzed by the following Equation S3: where is the capacitance, is the applied voltage, is the built-in potential, is the area of the perovskite, is the relative dielectric constant, 0 is the vacuum permittivity, is the donor concentration of perovskite [2] .

The Nyquist plots were measured with a ZAHNER Zennium Electrochemical
Workstation in the frequency range of 100 mHz to 4 MHz without external bias under dark condition.

Figure S1
The cross-sectional SEM images of the printable mesoscopic PSCs (a) and local enlarged images of m-TiO 2 /m-ZrO 2 (b) and carbon electrode (c).

Figure S2
The Surface and cross-sectional SEM images of the control device (a, d), the 3-CT treated device (b, e), and the 3-TEA treated device (c, f).

Figure S5
Cross-sectional scanning electron microscopy (SEM) of the 3-TEA treated device and energy-dispersive X-ray spectroscopy (EDX) analysis.

Figure S6
Cross-sectional SEM images of the 3-CT (a) and 3-TEA (b) treated devices based on FASnI 3 perovskite, and the corresponding EDX analysis. the Sn element represents the FTO and the FASnI 3 perovskite filled in the mesoscopic structure. The Ti, Zr and C element represents the m-TiO 2 , m-TiO 2 and porous carbon electrode, respectively.   Figure S10 UV-vis absorption spectra of the perovskite films with the different molecular treatment.

Figure S12
The UV-vis spectra (absorption pattern) of the different molecules treated devices stripped the carbon electrode. the absorbance of the 3-CT and 3-CT/3-TEA treated devices was not significantly different from that of the control, indicating that the 3-CT/3-TEA treatment did not impair the light absorption of the devices. In the printable mesoscopic devices, the light absorbing layer is the perovskite filled in m-TiO 2 . Previously, we proved that the 2D perovskites in the device are mainly distributed in the C electrode. Therefore, the 3-CT/3-TEA treatment of the device does not affect the light absorption of the solar cell.

Figure S18
The incident photon-to-current efficiency (IPCE) spectrums of different molecules treated devices.